1. Field of the Invention
The invention relates to a method for use in photochemical machining of titanium or zirconium substrates.
2. Description of the Prior Art
Titanium and zirconium, and alloys thereof, are used in various applications requiring superior resistance to corrosion and chemical attack. In many of these applications it would be advantageous to utilise photochemical machining to effect selective removal of material from the surface of a titanium or zirconium article. This practice is well known as an alternative to other metal removal methods, such as cutting or grinding, of various metals and alloys, including those of iron, nickle or cobalt base.
Titanium and zirconium and their alloys, however, are highly resistant to the conventional etching solutions used in photochemical machining. Effective etchants for titanium and zirconium are hydrofluoric acid and/or ammonium bifloride. Fluorine, being the most active of all the chemical elements, will attack most conventional photoresists used in photochemical machining to mask the areas which are not to be etched. Specifically, it has been found with the etching of titanium and zirconium substrates by these etchant solutions using a conventional photoresist, that during the etching reaction the etchant penetrates portions of the photoresist to result in etching of the substrate at unwanted areas. In addition, gas evolving from the etched substrate exerts pressure on the edges of the photoresist at the unmasked area to cause lifting thereof, which permits the etchant to penetrate between the photoresist and the substrate surface. This likewise causes unwanted etching of the substrate surface.